U.S. patent application number 12/175878 was filed with the patent office on 2009-01-22 for nonaqueous electrolyte secondary battery.
Invention is credited to Yoshiyuki Muraoka, Yukihiro OKADA.
Application Number | 20090023073 12/175878 |
Document ID | / |
Family ID | 40265101 |
Filed Date | 2009-01-22 |
United States Patent
Application |
20090023073 |
Kind Code |
A1 |
OKADA; Yukihiro ; et
al. |
January 22, 2009 |
NONAQUEOUS ELECTROLYTE SECONDARY BATTERY
Abstract
A nonaqueous electrolyte secondary battery includes a positive
electrode, a negative electrode, a porous insulating layer, and
nonaqueous electrolyte. The porous insulating layer is interposed
between the positive electrode and the negative electrode. The
nonaqueous electrolyte is contained at least in the porous
insulating layer. The mixture layer of the positive electrode and
the porous insulating layer each include a structure retainer.
Inventors: |
OKADA; Yukihiro; (Osaka,
JP) ; Muraoka; Yoshiyuki; (Osaka, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, NW
WASHINGTON
DC
20005-3096
US
|
Family ID: |
40265101 |
Appl. No.: |
12/175878 |
Filed: |
July 18, 2008 |
Current U.S.
Class: |
429/246 ;
429/231.95 |
Current CPC
Class: |
H01M 50/403 20210101;
H01M 50/431 20210101; H01M 4/625 20130101; H01M 50/446 20210101;
H01M 4/62 20130101; Y02T 10/70 20130101; H01M 10/052 20130101; H01M
50/46 20210101; H01M 4/13 20130101; H01M 4/139 20130101; Y02E 60/10
20130101; H01M 10/0525 20130101 |
Class at
Publication: |
429/246 ;
429/231.95 |
International
Class: |
H01M 2/14 20060101
H01M002/14; H01M 4/40 20060101 H01M004/40 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 18, 2007 |
JP |
2007-186589 |
Jul 8, 2008 |
JP |
2008-177608 |
Claims
1. A nonaqueous electrolyte secondary battery, comprising: a
positive electrode having a mixture layer which contains an active
material in which lithium ions are electrochemically intercalatable
and deintercalatable; a negative electrode having a layer which
contains an active material in which lithium ions are
electrochemically intercalatable and deintercalatable; a porous
insulating layer interposed between the positive electrode and the
negative electrode; and nonaqueous electrolyte contained at least
in the porous insulating layer, wherein the mixture layer of the
positive electrode includes a structure retainer, and the porous
insulating layer includes a structure retainer or is formed by a
structure retainer.
2. The nonaqueous electrolyte secondary battery of claim 1, wherein
the structure retainer prevents expansion of the active material of
the negative electrode in the thickness direction of the layer of
the negative electrode.
3. The nonaqueous electrolyte secondary battery of claim 1, wherein
the structure retainer included in the porous insulating layer is
ceramic granules.
4. The nonaqueous electrolyte secondary battery of claim 3, wherein
the ceramic granules have the shape of twigs.
5. The nonaqueous electrolyte secondary battery of claim 1, wherein
the structure retainer included in the mixture layer of the
positive electrode is ceramic granules having the shape of
twigs.
6. The nonaqueous electrolyte secondary battery of claim 1, wherein
the structure retainer included in the mixture layer of the
positive electrode is carbon fiber.
7. The nonaqueous electrolyte secondary battery of claim 1, wherein
the structure retainer is also included in the layer of the
negative electrode.
8. The nonaqueous electrolyte secondary battery of claim 7, wherein
the structure retainer included in the layer of the negative
electrode is carbon fiber.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a nonaqueous electrolyte
secondary battery.
[0003] 2. Description of the Prior Art
[0004] In recent years, shift of electronic devices into portable
and cordless forms has been explosive, and accordingly, demands for
compact, light-weighted, high energy density secondary batteries
for use as a battery for driving such electronic devices have been
increasing. Meanwhile, it has been proposed that a secondary
battery is used not only for compact consumer devices but also for
a power source for a device which need to have long-term
durability, such as a power source for power storage, a power
source for electric vehicles, etc. Broadening of the technical
fields to which secondary batteries are applicable has been
acceleratedly advancing.
[0005] Among various secondary batteries, nonaqueous electrolyte
secondary batteries, especially, lithium ion secondary batteries,
achieve higher voltages and higher energy densities. Therefore, the
nonaqueous electrolyte secondary batteries have been expected for
use as a power source for electronic devices, a power source for
power storage, or a power source for electric vehicles.
[0006] The nonaqueous electrolyte secondary battery includes a
positive electrode, a negative electrode, a separator, and
nonaqueous electrolyte. The separator is interposed between the
positive electrode and the negative electrode. The separator is
mainly formed by a polyolefin microporous membrane. The nonaqueous
electrolyte is contained at least in the separator. The nonaqueous
electrolyte is liquid nonaqueous electrolyte prepared by dissolving
lithium salt, such as LiBF.sub.4 or LiPF.sub.6, into an aprotic
organic solvent. Nonaqueous electrolyte secondary batteries have
been turned into practical uses, wherein the active material of the
positive electrode is an active material which is high in potential
relative to lithium and in which lithium ions are electrochemically
intercalatable and deintercalatable (for example, LiCoO.sub.2,
LiNiO.sub.2, LiMn.sub.2O.sub.4, LiFePO.sub.4) while the active
material of the negative electrode is any of various carbon
materials, such as graphite, or a metal oxide.
[0007] In such a nonaqueous electrolyte secondary battery, during
charging operation, lithium ions from the active material of the
positive electrode are intercalated between the crystal layers of
the active material of the negative electrode, while during
discharging operation, the lithium ions residing between the
crystal layers of the active material of the negative electrode
return to the active material of the positive electrode. Thus,
charging and discharging of the nonaqueous electrolyte secondary
battery cause the active material of the positive electrode and the
active material of the negative electrode to expand or
contract.
[0008] Specifically, the active material of the positive electrode
releases lithium ions during charging operation and regains the
lithium ions during discharging operation. Herein, the active
material of the positive electrode exists in the form of a lithium
oxide, a lithium phosphate, or a lithium sulfate. The crystal
structure of the lithium oxide, the lithium phosphate, or the
lithium sulfate has a robust framework, such as a layered rock-salt
structure, a spinel structure, or an olivine structure. Therefore,
expansion and contraction of the active material of the positive
electrode due to intercalation and deintercalation of lithium are
very small.
[0009] On the other hand, in the active material of the negative
electrode, lithium ion is intercalated between the crystal layers
of the active material of the negative electrode during charging
operation so that the space between the crystal layers is expanded.
Therefore, the charging operation causes the active material of the
negative electrode to expand. In the case where an alloy is used as
the active material of the negative electrode, the expansion of the
active material of the negative electrode is very large.
[0010] When the active material of the negative electrode expands,
there is a probability that the positive electrode and the
separator are compressed. When the positive electrode and the
separator are compressed, voids that exist in a mixture layer of
the positive electrode and in the separator are collapsed. Since
the voids contain the nonaqueous electrolyte, the collapse of the
voids causes the nonaqueous electrolyte to be expelled out of the
voids. As a result, as the cycles of charging/discharging are
repeated over and over, the battery capacity can decrease (the
cycle characteristics can deteriorate). Therefore, it is preferable
to prevent expansion of the active material of the negative
electrode during charging operation.
[0011] For the purpose of ameliorating the decrease in capacity
which could occur along with repeated charging/discharging cycles,
it has been proposed to mix Vapor Grown Carbon Fibers (VGCF) into
the negative electrode (see Japanese Laid-Open Patent Publication
No. 10-162811; hereinafter, this publication is referred to as
"Document 1"). Document 1 describes that the vapor grown carbon
fibers compensate for expansion and compression of the active
material of the negative electrode to prevent deformation of the
negative electrode. As a result, the decrease in capacity which
could occur along with repeated charging/discharging cycles can be
prevented.
SUMMARY OF THE INVENTION
[0012] However, even when the technique disclosed in Document 1 is
used, it is difficult to completely prevent expansion of the active
material of the negative electrode. Accordingly, as described
above, expansion of the active material of the negative electrode
causes the separator and the positive electrode to be compressed,
so that the voids exist in the separator and in the mixture layer
of the positive electrode are collapsed. As a result, there is a
probability that the nonaqueous electrolyte contained in the voids
is expelled out of the voids. Thus, the technique disclosed in
Document 1 still has the disadvantage of deteriorating the cycle
characteristics.
[0013] In the circumstances where higher capacity is demanded of
the nonaqueous electrolyte secondary batteries, an active material
which achieves higher capacity than carbon materials, such as
alloys, metal oxides, or Si oxides, is also demanded as the active
material of the negative electrode. When using such a high capacity
active material, more lithium ions can be intercalated between the
crystal layers of the active material of the negative electrode as
compared with an example where a carbon material, such as graphite,
is used as the active material. Accordingly, the expansion rate
during charging operation increases. Therefore, even the technique
disclosed in Document 1 cannot sufficiently prevent the expansion
of the active material of the negative electrode in some cases.
[0014] Recently, a negative electrode wherein the active material
of the negative electrode is vacuum-deposited over the surface of
the current collector of the negative electrode has been studied.
In this negative electrode, it is difficult to use the method
disclosed in Document 1.
[0015] In view of such circumstances, an objective of the present
invention is to solve such problems by providing a nonaqueous
electrolyte secondary battery having excellent cycle
characteristics.
[0016] Specifically, a nonaqueous electrolyte secondary battery of
the present invention includes a positive electrode, a negative
electrode, a porous insulating layer, and nonaqueous electrolyte.
The positive electrode has a mixture layer containing an active
material in which lithium ions are electrochemically intercalatable
and deintercalatable. The negative electrode has a layer containing
an active material in which lithium ions are electrochemically
intercalatable and deintercalatable (hereinafter, referred to as
"the active material layer of the negative electrode"). The porous
insulating layer is interposed between the positive electrode and
the negative electrode. The nonaqueous electrolyte is contained at
least in the porous insulating layer. The mixture layer of the
positive electrode includes a structure retainer. The porous
insulating layer includes a structure retainer or is formed by a
structure retainer.
[0017] In a nonaqueous electrolyte secondary battery of the present
invention, the porous insulating layer and the mixture layer of the
positive electrode include a structure retainer. Therefore, even
when the active material of the negative electrode is stimulated to
expand, the expansion of the active material is prevented. Namely,
the separator and the positive electrode are prevented from being
compressed by the expanded negative electrode. Thus, the nonaqueous
electrolyte is prevented from being expelled out of the voids that
exist in the separator and in the mixture layer of the positive
electrode, so that deterioration in cycle characteristics can be
prevented.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1A to FIG. 1C schematically illustrate the compressive
force produced by expansion of the active material of the negative
electrode during charging operation. FIG. 1A shows an example where
none of the structure retainer and the expansion inhibitor is
provided. FIG. 1B shows an example where only the expansion
inhibitor is provided in the active material layer of the negative
electrode. FIG. 1C shows an example where the structure retainer is
provided in both the mixture layer of the positive electrode and
the porous insulating layer.
[0019] FIG. 2 is a scanning electron microscopic image of ceramic
granules in the shape of twigs.
[0020] FIG. 3 shows a general vertical cross section of a
nonaqueous electrolyte secondary battery.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] Hereinafter, embodiments of the present invention are
described in detail with reference to the drawings. It should be
noted that the present invention is not limited to the embodiments
presented below.
[0022] A nonaqueous electrolyte secondary battery of this
embodiment includes a positive electrode, a negative electrode, a
porous insulating layer, and nonaqueous electrolyte. The positive
electrode has a mixture layer on one side of a current collector or
mixture layers on both sides of the current collector. The mixture
layer of the positive electrode contains an active material in
which lithium ions are electrochemically intercalatable and
deintercalatable, a binder, and a conductor. The negative electrode
has an active material layer(s) containing the active material on
one side or both sides of the current collector. The active
material of the negative electrode is a material in which lithium
ions are electrochemically intercalatable and deintercalatable. The
porous insulating layer is interposed between the positive
electrode and the negative electrode. The nonaqueous electrolyte is
contained in the positive electrode, the negative electrode and the
separator. The mixture layer of the positive electrode and the
porous insulating layer each include a structure retainer.
[0023] In the nonaqueous electrolyte secondary battery having such
a structure, the mixture layer of the positive electrode and the
porous insulating layer each include a structure retainer.
Therefore, even when the active material of the negative electrode
expands during charging operation, deformation of the positive
electrode and the porous insulating layer is prevented. As a
result, even if the active material of the negative electrode
expands, the nonaqueous electrolyte can be prevented from being
expelled from the voids of the mixture layer of the positive
electrode and the porous insulating layer, so that the
deterioration in cycle characteristics can be prevented.
[0024] The structure retainer is now described with reference to
FIG. 1A to FIG. 1C. FIG. 1A to FIG. 1C schematically illustrate
compressive force (the force that compresses the positive electrode
or the porous insulating layer) which is produced as a result of
expansion of the active material A of the negative electrode by
charging operation. FIG. 1A shows an example where none of an
expansion inhibitor for inhibiting expansion of the active material
A of the negative electrode and the structure retainer is provided.
FIG. 1B shows an example where an expansion inhibitor is provided
in the active material layer N of the negative electrode while no
structure retainer is provided. FIG. 1C shows an example where no
expansion inhibitor is provided while the structure retainers are
provided in the mixture layer P of the positive electrode and the
porous insulating layer I. In FIG. 1A to FIG. 1C, "Fx", "Fy" and
"Fz" denote the magnitude of compressive forces in x-direction,
y-direction and z-direction, respectively. The x-direction is a
direction perpendicular to the surface of the sheet of the drawing.
The y-direction is a lateral direction across the sheet of the
drawing. The z-direction is a vertical direction across the sheet
of the drawing.
[0025] Herein, the expansion inhibitor is, as disclosed in Document
1, a material which isotropically inhibits expansion of the active
material of the negative electrode. The structure retainer is a
material which prevents expansion of the active material of the
negative electrode in the thickness direction of the active
material layer of the negative electrode.
[0026] In general, when charging the nonaqueous electrolyte
secondary battery, lithium ions from the active material of the
positive electrode are intercalated between the crystal layers of
the active material of the negative electrode, resulting in
expansion of the active material of the negative electrode.
[0027] In the absence of the inhibitor and structure retainer, the
active material A of the negative electrode isotropically expands
as shown in FIG. 1A. Thus, Fx(1).apprxeq.Fy(1).apprxeq.Fz(1). Since
the active material A of the negative electrode expands in the
z-direction (the thickness direction of the active material layer
of the negative electrode) as shown in FIG. 1A, compressive force
(Fz(1).apprxeq.0) works on the separator I and the positive
electrode, so that the mixture layer P of the positive electrode
and the separator I are compressed. As a result of the compression,
the voids of the mixture layer P of the positive electrode and the
separator I for containing the nonaqueous electrolyte collapse, so
that the nonaqueous electrolyte is expelled out of the voids.
[0028] In the example described in Document 1 where the expansion
inhibitor is contained in the active material layer N of the
negative electrode, as shown in FIG. 1B, the expansion inhibitor
(not shown) isotropically inhibits expansion of the active material
A of the negative electrode. Therefore,
Fx(2).apprxeq.Fy(2).apprxeq.Fz(2).noteq.0, though Fx(2)<Fx(1),
Fy(2)<Fy(1), and Fz(2)<Fz(1). Thus, it is difficult to
inhibit the compression of the mixture layer P of the positive
electrode and the separator I.
[0029] In an example of the nonaqueous electrolyte secondary
battery of the present embodiment where the mixture layer P of the
positive electrode and the porous insulating layer I include the
structure retainers (not shown), even when compressive force is
applied to the mixture layer P and the porous insulating layer I of
the positive electrode, the retaining force for retaining the
structure of the mixture layer P of the positive electrode and the
porous insulating layer I works on the negative electrode against
the compressive force. Since the retaining force is transmitted
from the mixture layer P of the positive electrode and the porous
insulating layer I to the negative electrode in this way, expansion
of the active material A of the negative electrode in the
z-direction is prevented as shown in FIG. 1C (Fz(3).apprxeq.0). As
a result, the mixture layer P of the positive electrode and the
porous insulating layer I are prevented from being compressed.
Accordingly, the voids included in the mixture layer P of the
positive electrode and in the porous insulating layer I are
prevented from collapsing, so that the nonaqueous electrolyte is
prevented from being expelled out of the voids. Thus, deterioration
in cycle characteristics can be prevented.
[0030] Even when the expansion in the z-direction is thus
prevented, the active material of the negative electrode expands in
the x- and y-directions during charging operation such that the
voids of the active material layer of the negative electrode are
collapsed (Fx(3).noteq.0, Fy(3).noteq.0). Thus, in the nonaqueous
electrolyte secondary battery of this embodiment, collapse of the
mixture layer of the positive electrode and the separator can be
prevented without deteriorating the charging characteristics.
[0031] The expansion inhibitor of Document 1 isotropically inhibits
the expansion of the active material A of the negative electrode.
Therefore, in the example of FIG. 1B, namely, in the case where the
expansion inhibitor is included in the active material layer N of
the negative electrode as described in Document 1, complete
inhibition of the expansion of the active material A of the
negative electrode hinders lithium ions entering between the
crystal layers of the active material A of the negative electrode
during charging operation, resulting in a probability of
deterioration in charging characteristics.
[0032] Preferably, the structure retainer is also included in the
active material layer of the negative electrode. This prevents
expansion of the active material of the negative electrode, so that
compression of the mixture layer of the positive electrode and the
porous insulating layer is further prevented.
[0033] In summary, according to this embodiment, the structure
retainer is included in the mixture layer of the positive electrode
and in the porous insulating layer and, preferably, also included
in the active material layer of the negative electrode. With the
structure retainer included in the mixture layer of the positive
electrode and the porous insulating layer, the structure of the
mixture layer of the positive electrode and the porous insulating
layer is retained. As a result, the expansion of the active
material of the negative electrode in the thickness direction of
the active material layer of the negative electrode is prevented.
Therefore, in the mixture layer of the positive electrode and the
porous insulating layer, the structure retainer functions as a
compression-resistant component. On the other hand, when the
structure retainer is included in the active material layer of the
negative electrode, expansion of the active material of the
negative electrode is prevented. In other words, the structure
retainer in the active material layer of the negative electrode
functions as an expansion-resistant component.
[0034] The structure retainer is preferably included in both the
mixture layer of the positive electrode and the porous insulating
layer. This is because, if the structure retainer is only included
in the mixture layer of the positive electrode, the compressive
force produced by the expansion of the active material of the
negative electrode concentrates on the porous insulating layer, so
that the porous insulating layer is compressed in some cases.
Likewise, if the structure retainer is only included in the porous
insulating layer, the compressive force produced by the expansion
of the active material of the negative electrode concentrates on
the mixture layer, so that the mixture layer of the positive
electrode is compressed in some cases.
[0035] The shape of the structure retainer is preferably determined
such that the structure retainer solely is unlikely to cause
occlusion in the voids, i.e., such that the tap density is small.
The shape of the structure retainer is preferably, for example, a
shape with a high aspect ratio, a shape of tangled threads, a shape
of a twig or tetrapod. Since the structure retainers are entangled
with one another, the compression resistance of the mixture layer
of the positive electrode and the porous insulating layer can be
improved. Further, the expansion-resistivity of the active material
layer of the negative electrode can be improved. Specific examples
are described below.
[0036] The structure retainer included in the porous insulating
layer is preferably ceramic granules. The ceramic granules are
unlikely to be deformed even with application of external force and
can therefore improve the compression resistance of the porous
insulating layer. Also, the ceramic granules have excellent
insulation properties so that insulation can be secured between the
positive electrode and the negative electrode.
[0037] The shape of the ceramic granule is preferably the shape of
a twig. With the shape of a twig, the ceramic granules are
entangled with one another so that the compression resistance of
the porous insulating layer is further improved as compared with,
for example, spherical ceramic granules.
[0038] Herein, the porous insulating layer may include the
structure retainer or may be formed by the structure retainer. The
porous insulating layer formed by the structure retainer includes
not only an arrangement where the porous insulating layer is formed
only by the structure retainer but also an arrangement where the
structure retainers are adhesively combined together. For example,
if the structure retainers are ceramic granules as mentioned above,
the porous insulating layer may be formed by the ceramic granules
and an adhesive resin, polyethylene, polypropylene, or polyimide.
Alternatively, the porous insulating layer may be formed by a
plurality of ceramic granules adhered together.
[0039] The structure retainer included in the mixture layer of the
positive electrode may be ceramic granules having the shape of
twigs or may be carbon fiber. With the ceramic granules having the
shape of twigs, the ceramic granules are entangled with one another
as described above so that the compression resistance of the
mixture layer of the positive electrode can be improved. Note that
the present inventors confirmed that the power characteristics of
the battery can be secured even if the ceramic granules are
included in the mixture layer of the positive electrode although
the ceramic granules are insulative. We are uncertain of the reason
but estimate that the ceramic granules included in the mixture
layer of the positive electrode improve the compression resistance
of the mixture layer of the positive electrode, so that passageways
of the nonaqueous electrolyte can be secured in the mixture layer
of the positive electrode.
[0040] When using carbon fibers, the carbon fibers are entangled
with one another so that the compression resistance of the mixture
layer of the positive electrode can be improved. The carbon fiber
is excellent in electric conductivity so that the conductivity of
the mixture layer of the positive electrode can be secured.
[0041] The structure retainer included in the active material layer
of the negative electrode is preferably carbon fiber. The carbon
fibers entangled with one another can prevent expansion of the
active material of the negative electrode.
[0042] Specific material examples of the structure retainer are
described below.
[0043] When the ceramic granules are included as the structure
retainer in the porous insulating layer and in the mixture layer of
the positive electrode, the ceramic granules used are a single
species of metal oxide, metal nitride or metal carbonate, or a
combination of such materials. Among the metal oxide, metal nitride
and metal carbonate, the use of metal oxide is preferable because
of, for example, higher availability. Examples of the metal oxide
include alumina (aluminum oxide), titania (titanium oxide),
zirconia (zirconium oxide), magnesia (magnesium oxide), zinc oxide,
and silica (silicon oxide). Among these examples of the metal
oxide, alumina is preferable, and .alpha.-alumina is more
preferable. .alpha.-alumina is chemically stable, and
.alpha.-alumina with high purity is chemically very stable.
"Chemically stable" means, for example, that it is not corroded by
the nonaqueous electrolyte even if it comes in contact with the
nonaqueous electrolyte, that it is not decomposed or brought into
reaction even if an oxidation-reduction potential is applied to the
positive electrode and the negative electrode, and that a side
reaction does not occur which would cause an adverse effect on the
battery characteristics.
[0044] Preferably, the ceramic granules have the average
monocrystalline grain size of about 0.05 .mu.m to 1 .mu.m. The
shape of the ceramic granule may be a sphere formed by general
primary particles aggregated by van der Waals force, but desirably
be a shape having an engagement portion by which monocrystalline
cores are engaged with one another as shown in FIG. 2. FIG. 2 shows
that grown monocrystalline cores of the ceramic granules are
engaged together. A polycrystalline grain 21 is not spherical but
generally has bumps, upheavals, or bulges, and preferably has the
shape of a twig, a staghorn or a cluster. The ceramic granule
preferably has a neck in the engagement portion for engaging
monocrystalline cores together (see FIG. 2). However, even a
granule whose neck is not clearly identified can be used.
[0045] Such ceramic granules containing polycrystalline grains are
produced through the step of baking a ceramic precursor to obtain
ceramic chips and the step of mechanically shredding the ceramic
chips into granules. The ceramic chips have a structure of grown
monocrystalline cores that are three-dimensionally linked together.
Such ceramic chips are mechanically and properly shredded to obtain
ceramic granules containing polycrystalline grains. The ceramic
granules are preferably formed by polycrystalline grains but may
contain another type of grains, for example, spherical or
substantially spherical primary particles, or spherical grains
formed by primary particles aggregated by van der Waals force, at
the ratio of, for example, less than 30% by weight.
[0046] The porous insulating layer is preferably formed by the
ceramic granules and the binder. In this case, the binder may be,
for example, a fluorine resin. Examples of the fluorine resin
include PolyVinylidene Fluoride (PVDF), polytetrafluoroethylene
(PTFE) and tetrafluoroethylene-hexafluoropropylene copolymer (FEP;
Fluorinated-Ethylene-Propylene). The binder may be a polyacrylic
acid derivative or a polyacrylonitrile derivative. The polyacrylic
acid derivative and the polyacrylonitrile derivative include, in
addition to an acrylic acid unit and an acrylonitrile unit, any one
of the group consisting of a methyl acrylate unit, an ethyl
acrylate unit, a methyl methacrylate unit and an ethyl methacrylate
unit. The binder may be polyethylene or styrene-butadiene rubber.
The binder may be any one of these materials solely used or may be
a combination of two or more of these materials. Among these
materials, a polymer containing an acrylonitrile unit, i.e., a
polyacrylonitrile derivative, is especially preferable. With any of
these materials used as the binder, the porous insulating layer can
be more flexible so that occurrence of cracks in the porous
insulating layer can be prevented, and the porous insulating layer
can be prevented from peeling off the positive electrode or the
negative electrode.
[0047] The void ratio in the porous insulating layer is preferably
30% to 80%, more preferably 40% to 80%, more preferably 50% to 80%.
The porous insulating layer preferably has the void ratio of 30% or
more, especially 40% or more, because deterioration in the
charge/discharge characteristics due to a large current flowed in
the battery and deterioration in the charge/discharge
characteristics in a low-temperature environment can be prevented.
However, the void ratio higher than 80% is not preferable because
the mechanical strength of the porous insulating layer is low.
[0048] An example where the mixture layer of the positive electrode
contains carbon fiber as the structure retainer is now described.
Examples of the carbon fiber include vapor grown carbon fiber
(VGCF), carbon fiber, and carbon nanotube. The vapor grown carbon
fiber is generally in an aggregated form, and therefore, this
aggregate of carbon fiber is used as the structure retainer. The
aggregate is formed by aggregated vapor grown carbon fibers each
having a fiber diameter of 0.01 to 5 .mu.m, wherein some of
contacts of the vapor grown carbon fibers are chemically adhered by
a carbide of an organic compound, such as tar, pitch, or the like,
to form a floc or bobble having the diameter of 5 to 500 .mu.m. The
inside of the aggregate includes very small voids of different
sizes. Such aggregate can be prepared using any one of the
following methods. The first method includes first compressing
vapor grown carbon fibers with a fiber diameter of 0.05 to 5 .mu.m
to form a molded element with the bulk density of 0.02 g/cm.sup.3
or more, and then heating the molded element to 600.degree. C.,
preferably 800.degree. C. or higher, and shredding the heated
molded element. The second method includes heating vapor grown
carbon fibers to 600.degree. C. or higher, preferably 800.degree.
C. or higher, while compressing the fibers with the pressure of 0.1
kg/cm.sup.2 or more, and then shredding the heated molded element
(see Japanese Laid-Open Patent Publication No. 9-132846). The vapor
grown carbon fibers are not limited to any particular type but may
be, for example, filaments with no branching, or may be fibers with
branching, or may be combination of filaments and fibers with
branching, but are preferably coarse fibers as produced, i.e.,
thermally untreated. The coarse carbon fibers have about 5 to 20
percents by weight of tar or pitch adsorbed thereon. The tar or
pitch functions as a binder for binding carbon fibers when the
carbon fibers are compressed, and is readily carbonized when heated
to adhere carbon fibers together. When using thermally-treated
vapor grown carbon fibers, the carbon fibers may desirably be
molded with addition of pitch, or the like.
[0049] Preferably, such carbon fibers are also included in the
active material layer of the negative electrode as described
above.
[0050] Hereinafter, the structures of the positive electrode, the
negative electrode and the nonaqueous electrolyte are described in
this order.
[0051] As described above, the positive electrode includes mixture
layers on both sides of the current collector of the positive
electrode or includes a mixture layer on one side of the current
collector. The mixture layers of the positive electrode contain an
active material for the positive electrode in which at least
lithium ions are electrochemically intercalatable and
deintercalatable, a binder, a conductor, and a structure
retainer.
[0052] Examples of the active material in which lithium ions are
electrochemically intercalatable and deintercalatable includes
oxides, phosphates, sulfates, and silicates that contain lithium
and any metal element other than lithium. The any metal element
other than lithium may be one element or may be one or more
elements. Specifically, preferable examples of the active material
in which lithium ions are electrochemically intercalatable and
deintercalatable includes lithium cobaltate (LiCoO.sub.2),
denatured lithium cobaltate, lithium nickelate (LiNiO.sub.2),
denatured lithium nickelate, lithium manganate (LiMn.sub.2O.sub.2),
denatured lithium manganate, lithium iron phosphate (LiFePO.sub.4),
denatured lithium iron phosphate, lithium manganese phosphate
(LiMnPO.sub.4), denatured lithium manganese phosphate, and lithium
iron sulfate (LiFeSO.sub.4). The active material of the positive
electrode may be any of the aforementioned compounds in which part
of Co, Ni, Mn or Fe is replaced by another transition metal element
or main group metal element (e.g., aluminum or magnesium).
[0053] The binder of the positive electrode is not limited to any
particular type of material but may be polytetrafluoroethylene
(PTFE), denatured PTFE, PVDF, denatured PVDF, or denatured
acrylonitrile rubber particles. When using PTFE or denatured
acrylonitrile rubber particle binder (for example, BM-500B
manufactured by ZEON Corporation) as the binder,
carboxymethylcellulose (CMC), polyethylene oxide (PEO), or
denatured acrylonitrile rubber is preferably used together as the
thickener.
[0054] Examples of the conductor of the positive electrode include
acetylene black, ketjen black, and various graphites. Any one of
these examples may be solely used. Alternatively, two or more of
these examples may be used in combination.
[0055] The current collector of the positive electrode preferably
contains a stable metal which is not decomposed or molten even when
a positive electrode potential is applied thereto. For example, the
current collector may be realized by an aluminum foil or a film
which has metal, such as aluminum, on the surface of the film. The
current collector may have an undulated surface or may have
bores.
[0056] The negative electrode has, as described above, the active
material layers on both sides or one side of the current collector
of the negative electrode. The active material layer of the
negative electrode may contain the active material of the negative
electrode and the binder. The active material layer of the negative
electrode may further contain a thickener. The active material
layer of the negative electrode may further contain a structure
retainer.
[0057] The active material of the negative electrode is not limited
to any particular material so long as it is capable of
electrochemical intercalation and deintercalation of lithium ions.
Specifically, examples of the active material of the negative
electrode include various natural graphites, various artificial
graphite, petroleum cokes, carbon fibers, carbon materials, such as
baked organic polymers, metal oxides, silicon, silicon-containing
composite materials (materials containing silicon and at least one
element other than silicon), tin-containing composite materials
(materials containing tin and at least one element other than tin),
and various metals and alloys.
[0058] The binder of negative electrode is not limited to any
particular type of material but is preferably rubber particles
because even a small amount of rubber particles can achieve
excellent binding property. Specifically, rubber particles
containing a styrene group and a butadiene group are preferable.
Examples of the binder include styrene-butadiene copolymer (SBR;
styrene-butadiene rubber), denatured SBR, etc. In the case where
rubber particles are used as the binder of the negative electrode,
a thickener composed of a water-soluble polymer is desirably used
together. The water-soluble polymer is preferably a cellulose
resin, more preferably, CMC. Preferably, the amounts of the binder
and the thickener in the active material layer of the negative
electrode are each 0.1 to 5 parts by weight relative to 100 parts
by weight of the active material of the negative electrode.
Alternative examples of the binder include PVDF and denatured
PVDF.
[0059] Alternatively, in the negative electrode, the active
material may be precipitated over the surface of the current
collector of the negative electrode using a vapor deposition
method.
[0060] The current collector of the negative electrode preferably
contains a stable metal which is not decomposed or molten even when
a negative electrode potential is applied thereto. For example, the
current collector may be realized by a copper foil or a film which
has metal, such as copper, on the surface of the film. The current
collector may have an undulated surface or may have bores.
[0061] The nonaqueous electrolyte is composed of an organic
nonaqueous solvent and lithium salt dissolved therein. The
concentration of the lithium salt is generally 0.5 to 2 mol/L.
[0062] The lithium salt is not limited to any particular type but
is preferably lithium phosphate hexafluoride (LiPF.sub.6), lithium
perchlorate (LiClO.sub.4), lithium fluoroborate (LiBF.sub.4), or
the like. As the lithium salt, any one of these examples may be
solely used. Alternatively, two or more of these examples may be
used in combination.
[0063] The organic nonaqueous solvent is not limited to any
particular type. Preferable examples of the organic nonaqueous
solvent, however, include: carbonic esters, such as ethylene
carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC),
diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the
like; carboxylate esters, such as .gamma.-butyrolactone,
.gamma.-valerolactone, methyl formate, methyl acetate, methyl
propionate, and the like; and ethers, such as dimethyl ether,
diethyl ether, tetrahydrofuran, and the like. As the organic
nonaqueous solvent, any one of these examples may be solely used.
Alternatively, two or more of these examples may be used in
combination. Among these examples, especially, carbonic esters are
preferably used.
[0064] To form an excellent film over the electrode such that the
stability is secured even when overcharged, adding Vinylene
Carbonate (VC), cyclohexylbenzene (CHB), or denatured VC or CHB to
the nonaqueous electrolyte is preferable.
[0065] Although it is described above that the mixture layer of the
positive electrode preferably contains ceramic granules or carbon
fibers as the structure retainer in this embodiment, the mixture
layer of the positive electrode may contain both ceramic granules
and carbon fibers.
[0066] Although the structure of an electrode assembly is not
specifically described, the electrode assembly of this embodiment
may be formed by rolling up the positive electrode and the negative
electrode together with the porous insulating layer interposed
therebetween. Alternatively, the electrode assembly may be formed
by combining the positive electrode and the negative electrode into
a laminate with the porous insulating layer interposed
therebetween. When the positive electrode and the negative
electrode are rolled up together, the shape of an end face of the
resultant electrode assembly may be a circle or may be a flattened
circle. In fabrication of such an electrode assembly, the porous
insulating layer may be interposed between the positive electrode
and the negative electrode when they are rolled up or combined into
a laminate. Alternatively, paste containing the constituents of the
porous insulating layer may be applied over a surface of the
positive electrode or the negative electrode and dried before the
positive electrode and the negative electrode are rolled up or
combined into a laminate.
[0067] Although the nonaqueous electrolyte solution is described as
an example of electrolyte, the aforementioned effects of the
present invention can, as a matter of course, be achieved with a
gel electrolyte.
[0068] Hereinafter, examples of the present invention are
specifically described with reference to the drawings. It should be
noted that the followings are merely examples of the present
invention, to which the present invention is not limited.
Example 1
(I) Preparation of Ceramic Granules Containing Polycrystalline
Grains
[0069] In connection with Example 1, we prepared ceramic granules
containing .alpha.-alumina.
[0070] First, aluminum tributoxide (aluminum alkoxide) was
prepared. The aluminum tributoxide was hydrolytically mixed in pure
water to produce alumina gel which was then dried. The dried
alumina gel was used as a ceramic precursor.
[0071] Then, the alumina gel (ceramic precursor) was baked at
1200.degree. C. for 3 hours to produce baked .alpha.-alumina chips
(baked ceramic chips). The resultant baked .alpha.-alumina chips
were SEM (scanning electron microscope) photographed, and the
average particle size of monocrystalline cores of .alpha.-alumina
was determined from the SEM photographic image, which was about 0.2
.mu.m.
[0072] The resultant baked .alpha.-alumina chips were shredded by a
jet mill. The shredding conditions were set such that the bulk
density of ceramic granules was 0.05 to 1.1 g/cm.sup.3, and the BET
specific surface was 3 to 22 m.sup.2/g.
[0073] The bulk density of .alpha.-alumina was measured by a
stationary method using "Powder Tester (trade name)" manufactured
by Hosokawa Micron. The BET specific surface of .alpha.-alumina was
measured using BET (Brunauer-Emmett-Teller) method. The resultant
granules on the SEM image were observed and found to be
polycrystalline particles in the shape of twigs.
(II) Preparation of Slurry Containing Constituents of Porous
Insulating Layer
[0074] Into 100 parts by weight of polycrystalline alumina
particles of a certain type, 4 parts by weight of polyacrylic acid
derivative ("BM-720H (trade name)" manufactured by ZEON
Corporation, as the binder) and an appropriate amount of
N-methyl-2-pyrrolidone (hereinafter, NMP; N-methylpyrrolidone, as
dispersion medium) were mixed to prepare slurry whose nonvolatile
components were 60% by weight.
[0075] In this example, a mixture of the polycrystalline alumina
particles, the binder, and the dispersion medium was stirred using
a medialess disperser "CLEARMIX (trade name)" manufactured by
M-technique Corporation. The mixture was stirred till the
polycrystalline alumina particles and the binder were uniformly
dispersed in the NMP to prepare slurry.
[0076] <a> Preparation of Positive Electrode
[0077] 3 kg of lithium cobaltate (the active material of the
positive electrode), 1 kg of "#1320 (trade name)" manufactured by
KUREHA Corporation (NMP solution containing 12% by weight of PVDF,
as the binder of the positive electrode), 90 g of acetylene black
(conductor), 60 g of VGCF (structure retainer) manufactured by
SHOWA DENKO K.K., and an appropriate amount of NMP were stirred
using a twin arm kneader to prepare a mixture coating material for
the positive electrode. Aluminum foil having a thickness of 15
.mu.m (for the current collector of the positive electrode) was
prepared and coated with the mixture coating material for the
positive electrode to form a coating layer over the aluminum foil
except for part of the foil to which a lead wire of the positive
electrode was to be connected (connection portion for positive
electrode lead wire). After dried, the dried coating layer was
rolled using a roller. As a result, a mixture layer for the
positive electrode was formed over the aluminum foil so that the
density of the active material (=the weight of the active
material/the volume of the mixture layer) was 3.3 g/cm.sup.3. The
formation of this structure was controlled such that the total of
the thickness of the aluminum foil and the thickness of the mixture
layer for the positive electrode (the thickness of the positive
electrode) was 160 .mu.m. Then, the resultant structure was cut so
as to have a width insertable into a battery can of a cylindrical
battery (product No. 18650) and rolled into a hoop for the positive
electrode.
[0078] <b> Preparation of Negative Electrode
[0079] 3 kg of artificial graphite (the active material of the
negative electrode), 75 g of "BM-400B (trade name)" manufactured by
ZEON Corporation (aqueous dispersion containing 40% by weight of
denatured styrene-butadiene copolymer, as the binder), 30 g of CMC
(thickener), and an appropriate quantity of water were stirred by a
twin arm kneader to prepare a mixture coating material for the
negative electrode. Copper foil having a thickness of 10 .mu.m was
prepared and coated with the mixture coating material for the
positive electrode to form a coating layer over the copper foil
except for part of the foil to which a lead wire of the negative
electrode was to be connected (connection portion for negative
electrode lead wire). After dried, the dried coating layer was
rolled using a roller. As a result, the active material layer for
the negative electrode was formed over the copper foil so that the
density of the active material (=the weight of the active
material/the volume of the active material layer) was 1.4
g/cm.sup.3. The formation of this structure was controlled such
that the total of the thickness of the copper foil and the
thickness of the active material layer for the negative electrode
(the thickness of the negative electrode) was 180 .mu.m. Then, the
resultant structure was cut so as to have a width insertable into a
battery can of a cylindrical battery (product No. 18650) and rolled
into a hoop for the negative electrode.
[0080] <c> Formation of Porous Insulating Layer
[0081] Slurry containing the constituents of the porous insulating
layer was applied over the both sides of the hoop for the negative
electrode using a gravure roll method at the rate of 0.5 m/min.
Then, hot air at 120.degree. C. was applied to the slurry with the
flow rate of 0.5 m/sec for drying the slurry. As a result, the
porous insulating layer having the thickness of 20 .mu.m was formed
on each side of the negative electrode.
[0082] <d> Preparation of Nonaqueous Electrolyte Solution
[0083] LiPF.sub.6 was dissolved at the concentration of 1 mol/L in
a nonaqueous solvent mixture containing EC, DMC and EMC at the
volume ratio of 2:3:3 to prepare a nonaqueous electrolyte solution.
Further, 2 parts by weight of VC was added to 100 parts by weight
of the nonaqueous electrolyte solution.
[0084] <e> Fabrication of Battery
[0085] A cylindrical battery of product No. 18650 was fabricated as
described below using the positive electrode, the negative
electrode and the nonaqueous electrolyte solution which are
described above. FIG. 3 shows a general vertical cross section of
the nonaqueous electrolyte secondary battery.
[0086] First, the positive electrode hoop and the negative
electrode hoop with the porous insulating layers formed on both
sides thereof were cut so as to have predetermined lengths to
prepare a positive electrode plate 1 and a negative electrode plate
2 with porous insulating layers 3 formed on both sides thereof.
[0087] Then, an end of the positive electrode lead wire 5 was
resistance-welded to the connection portion for positive electrode
lead wire, and an end of the negative electrode lead wire 6 was
resistance-welded to the connection portion for negative electrode
lead wire. Thereafter, the positive electrode plate 1 and the
negative electrode plate 2 were arranged such that the porous
insulating layer 3 was sandwiched by the positive electrode plate 1
and the negative electrode plate 2, and the positive electrode
plate 1 and the negative electrode plate 2 were rolled up to form a
cylindrical electrode assembly 4. The thickness of the porous
insulating layer 3 of the resultant electrode assembly 4 was 20
.mu.m.
[0088] The negative electrode lead wire 6 was passed through a
lower insulation ring 10 before the electrode assembly 4 was put
into a metal closed-end case (battery case) 7. Then, the negative
electrode lead wire 6 was resistance-welded to the bottom of the
metal closed-end case 7 such that the negative electrode lead wire
6 was electrically connected to the closed-end case 7. An upper
insulation ring 9 was placed on the top end face of the electrode
assembly 4, and the positive electrode lead wire 5 was passed
through the upper insulation ring 9. Then, a neck was formed in the
vicinity of the open end of the metal closed-end case 7.
[0089] Subsequently, the positive electrode lead wire 5 was
laser-welded to a metal filter 8a of a sealing plate 8 which has an
outer gasket 11 made of a resin such that the positive electrode
lead wire 5 was electrically connected to the sealing plate 8.
Then, the nonaqueous electrolyte solution was poured into the metal
closed-end case 7 via its open end, and the pressure inside the
case 7 was decreased to 133 Pa so that the electrode assembly 4 was
impregnated with the nonaqueous electrolyte solution.
[0090] Then, the positive electrode lead wire 5 was bent, and the
sealing plate 8 that has the resin outer gasket 11 was attached to
the neck of the closed-end case 7 and caulked to seal the open end
of the closed-end case 7. In this way, a cylindrical nonaqueous
electrolyte secondary battery A was fabricated.
Example 2
[0091] A nonaqueous electrolyte secondary battery B was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
A of Example 1 except that VGCF was added as the structure retainer
to the active material layer of the negative electrode.
Specifically, 60 g of VGCF manufactured by SHOWA DENKO K.K. was
added to the mixture coating material for the negative electrode of
Example 1 to prepare the mixture coating material for the negative
electrode of Example 2.
Example 3
[0092] A nonaqueous electrolyte secondary battery C was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
A of Example 1 except that ceramic granules containing
polycrystalline grains were used in the mixture layer of the
positive electrode as the structure retainer instead of VGCF.
Specifically, 60 g of ceramic granules prepared as described in the
above section of Example 1, "(I) Preparation of Ceramic Granules
containing Polycrystalline Grains", were added instead of 60 g of
VGCF manufactured by SHOWA DENKO K.K. to prepare the mixture
coating material for the positive electrode of Example 3.
Example 4
[0093] A nonaqueous electrolyte secondary battery D was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
A of Example 1 except that alumina particles composed of spherical
or generally spherical primary particles having the average
particle size of 0.3 .mu.m were used as a constituent of the porous
insulating layer instead of the polycrystalline particles in the
shape of twigs.
Comparative Example 1
[0094] A nonaqueous electrolyte secondary battery E was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
A of Example 1 except that the structure retainer was not included
in the mixture layer of the positive electrode and that a
microporous membrane (the separator) made of a polyethylene resin
was used instead of the porous insulating layer. The method for
fabricating the battery E is described below.
[0095] <e> Fabrication of Battery
[0096] A cylindrical battery of product No. 18650 was fabricated as
described below using the positive electrode, the negative
electrode and the nonaqueous electrolyte solution which are
described above.
[0097] First, the positive electrode and the negative electrode
were cut so as to have predetermined lengths. Then, an end of the
positive electrode lead wire was connected to the connection
portion for positive electrode lead wire, and an end of the
negative electrode lead wire was connected to the connection
portion for negative electrode lead wire.
[0098] Then, the positive electrode and the negative electrode were
rolled up with the microporous membrane made of a polyethylene
resin having a thickness of 20 .mu.m interposed therebetween to
form a cylindrical electrode assembly.
[0099] Subsequently, a nonaqueous electrolyte secondary battery E
was fabricated in the same way as Example 1.
Comparative Example 2
[0100] A nonaqueous electrolyte secondary battery F was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
E of Comparative Example 1 except that VGCF was included as the
structure retainer in the active material layer of the negative
electrode. Namely, the negative electrode of Comparative Example 2
is the same as the negative electrode of Example 2.
Comparative Example 3
[0101] A nonaqueous electrolyte secondary battery G was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
E of Comparative Example 1 except that VGCF was included as the
structure retainer in the mixture layer of the positive electrode.
Namely, the positive electrode of Comparative Example 3 is the same
as the positive electrode of Example 1.
Comparative Example 4
[0102] A nonaqueous electrolyte secondary battery H was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
A of Example 1 except that the structure retainer was not included
in the mixture layer of the positive electrode, i.e., VGCF
(structure retainer) manufactured by SHOWA DENKO K.K. was not
included in the positive electrode. Namely, the positive electrode
of Comparative Example 4 is the same as the positive electrode of
Comparative Example 1.
Example 5
[0103] A nonaqueous electrolyte secondary battery I was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
A of Example 1 except that Si was used as the active material of
the negative electrode instead of the artificial graphite, that the
porous insulating layers were formed on both sides of the positive
electrode instead of the both sides of the negative electrode, and
that the nonaqueous electrolyte solution was prepared without
adding VC. Specifically, the negative electrode, the porous
insulating layer, and the battery were fabricated according to
fabrication methods described below.
[0104] <b> Fabrication of Negative Electrode
[0105] A single ingot of Si (manufactured by Kojundo Chemical Lab.
Co., Ltd.; Purity 99.999%; Average particle size 5 to 35 mm) was
put in a graphite crucible. This crucible was put in a vacuum
deposition apparatus, and electron beam vacuum deposition
(acceleration voltage: -8 kV, current: 150 mA, degree of vacuum:
3.times.10.sup.-5 Torr) was performed on an electrolytically
Cu-plated foil (manufactured by FURUKAWA CIRCUIT FOIL Co., Ltd.;
thickness 20 .mu.m; as the current collector of the negative
electrode). After deposition of Si on one side of the
electrolytically Cu-plated foil was completed, the vacuum
deposition was also performed on the other side of the
electrolytically Cu-plated foil (untreated surface) in the same
way. Thereby, the active material layers of the negative electrode
were formed on both sides of the electrolytically Cu-plated foil.
Thereafter, the electrode plate was slit so as to have a width
insertable into a battery can of a cylindrical battery (product No.
18650) and rolled into a hoop for the negative electrode.
[0106] <c> Formation of Porous Insulating Layer
[0107] Slurry containing the constituents of the porous insulating
layer was applied over the both sides of the positive electrode
hoop using a gravure roll method at the rate of 0.5 m/min. Then,
hot air at 120.degree. C. was applied to the slurry with the flow
rate of 0.5 m/sec for drying the slurry. As a result, the porous
insulating layer having the thickness of 20 .mu.m was formed on
each side of the positive electrode.
Comparative Example 5
[0108] A nonaqueous electrolyte secondary battery J was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
I of Example 5 except that a microporous membrane made of a
polyethylene resin was used instead of the porous insulating layer.
Specifically, the positive electrode was fabricated in the same way
as Example 1, the negative electrode was fabricated in the same way
as Example 5, and the battery was fabricated in the same way as
Comparative Example 1.
Example 6
[0109] A nonaqueous electrolyte secondary battery K was fabricated,
which was the same as the nonaqueous electrolyte secondary battery
B of Example 2 except that a microporous membrane made of a
polyethylene resin and containing alumina particles was used
instead of the porous insulating layer of Example 2. The
polyethylene resin microporous membrane containing alumina
particles was prepared using a method disclosed in Japanese
Laid-Open Patent Publication No. 2005-71979. A method for
fabricating the battery K is described below.
[0110] <e> Fabrication of Battery
[0111] A cylindrical battery of product No. 18650 was fabricated
through a procedure described below using the positive electrode,
the negative electrode and the nonaqueous electrolyte solution
which are described above. First, the positive electrode and the
negative electrode were cut so as to have predetermined lengths.
Then, an end of the positive electrode lead wire was connected to
the connection portion for positive electrode lead wire, and an end
of the negative electrode lead wire was connected to the connection
portion for negative electrode lead wire. Then, the positive
electrode and the negative electrode were rolled up with a
polyethylene resin microporous membrane, having a thickness of 20
.mu.m and containing ceramic granules, interposed therebetween to
form a cylindrical electrode assembly. Subsequently, the nonaqueous
electrolyte secondary battery K was fabricated in the same way as
Example 1.
[0112] For the nonaqueous electrolyte secondary batteries A to D, I
and K of the Examples and the nonaqueous electrolyte secondary
batteries E to H and J of the Comparative Examples, the number of
cycles was measured according to a procedure described below.
[0113] Each battery was subjected to preparatory
charging/discharging operation twice. After that, the battery was
charged and stored in an environment at 45.degree. C. for 7 days.
Subsequently, the battery was subjected to the charging/discharging
cycle described below in an environment at 20.degree. C.
[0114] <Cycle Conditions>
[0115] A current of 1000 mA was flowed to charge the battery till
the voltage of the battery reached 4.2 V (constant current
charging). During the next 1 minute, the battery was left alone.
Then, a current of 1000 mA was flowed to discharge the battery till
the voltage of the battery reached 3.0 V (constant current
discharging). During the next 1 minute, the battery was left
alone.
[0116] The results of measurement of the number of cycles is shown
in Table 1. It should be noted that the "Number of Cycles" in Table
1 refers to the number of cycles that elapsed before the discharge
capacity decreased below 80% of the initial discharge capacity.
TABLE-US-00001 TABLE 1 Active Structure Retainer Material of
Positive Negative Porous Insulating Porous Insulating Negative
Number of Electrode Electrode Layer Layer Electrode Cycles Example
1 VGCF none .alpha.-alumina/ .alpha.-alumina/ artificial 460
(Battery A) shape of twigs shape of twigs graphite Example 2 VGCF
VGCF .alpha.-alumina/ .alpha.-alumina/ artificial 540 (Battery B)
shape of twigs shape of twigs graphite Example 3 .alpha.-alumina/
none .alpha.-alumina/ .alpha.-alumina/ artificial 500 (Battery C)
shape of twigs shape of twigs shape of twigs graphite Example 4
VGCF none .alpha.-alumina/ .alpha.-alumina/ artificial 360 (Battery
D) primary particles primary particles graphite Comparative None
none none none artificial 220 Example 1 (polyethylene graphite
(Battery E) separator) Comparative None VGCF none none artificial
260 Example 2 (polyethylene graphite (Battery F) separator)
Comparative VGCF none none none artificial 180 Example 3
(polyethylene graphite (Battery G) separator) Comparative None none
.alpha.-alumina/ .alpha.-alumina/ artificial 240 Example 4 shape of
twigs shape of twigs graphite (Battery H) Example 5 VGCF none
.alpha.-alumina/ .alpha.-alumina/ Si 160 (Battery I) shape of twigs
shape of twigs Comparative VGCF none none none Si 60 Example 5
(polyethylene (Battery J) separator) Example 6 VGCF VGCF
.alpha.-alumina .alpha.-alumina artificial 500 (Battery K) (in
polyethylene graphite separator)
[0117] Comparing the measurement results of the nonaqueous
electrolyte secondary batteries A to H and K, the nonaqueous
electrolyte secondary batteries A to D and K of the Examples
achieved greater numbers of cycles than the nonaqueous electrolyte
secondary batteries E to H of the Comparative Examples. Below, the
nonaqueous electrolyte secondary batteries E to H of the
Comparative Examples are analyzed, and then, the nonaqueous
electrolyte secondary batteries A to D and K of the Examples are
analyzed.
[0118] The nonaqueous electrolyte secondary battery F of
Comparative Example 2 includes the structure retainer in the active
material layer of the negative electrode and therefore achieved a
greater number of cycles than the nonaqueous electrolyte secondary
battery E (Comparative Example 1). However, the effect (the rate of
improvement in number of cycles) was relatively small as compared
with the nonaqueous electrolyte secondary batteries A to D and K of
the Examples. Similarly, the nonaqueous electrolyte secondary
battery G of Comparative Example 3 includes the structure retainer
in the mixture layer of the positive electrode, and the nonaqueous
electrolyte secondary battery H of Comparative Example 4 includes
the structure retainer in the porous insulating layer. Therefore,
the battery G and the battery H achieved greater numbers of cycles
than the nonaqueous electrolyte secondary battery E. However, the
effect of improvement was relatively small as compared with the
nonaqueous electrolyte secondary batteries A to D and K of the
Examples. A conceivable reason for this is that, when the
nonaqueous electrolyte secondary battery is charged/discharged, the
active material of the positive electrode and the porous insulating
layer do not expand or contract, or do not substantially expand or
contract, while the active material of the negative electrode
expands or contracts more than the active material of the positive
electrode and the porous insulating layer do.
[0119] Conceivable reasons why the nonaqueous electrolyte secondary
batteries E to H achieved smaller numbers of cycles are described
below.
[0120] In the nonaqueous electrolyte secondary battery E, when the
active material of the negative electrode expands, the positive
electrode and the separator are compressed, so that the voids of
the positive electrode and the separator collapse. As a result, the
nonaqueous electrolyte solution is expelled out of the voids.
[0121] In the nonaqueous electrolyte secondary battery F, the
structure retainer prevents expansion of the active material of the
negative electrode, but it is difficult to sufficiently prevent the
expansion of the active material of the negative electrode.
Therefore, when the active material of the negative electrode
expands, the positive electrode and the separator are compressed.
As a result, the nonaqueous electrolyte solution is expelled out of
the mixture layer of the positive electrode and the separator.
[0122] In the nonaqueous electrolyte secondary battery G, the
structure retainer is included in the mixture layer of the positive
electrode. However, when the active material of the negative
electrode expands, the separator is compressed so that the
nonaqueous electrolyte solution is expelled out of the
separator.
[0123] The nonaqueous electrolyte secondary battery H includes the
porous insulating layer instead of the separator, and the porous
insulating layer includes the structure retainer. However, the
structure retainer is not included in the mixture layer of the
positive electrode. Therefore, when the active material of the
negative electrode expands, the positive electrode is compressed.
As a result, the nonaqueous electrolyte solution is expelled out of
the mixture layer of the positive electrode.
[0124] On the other hand, the nonaqueous electrolyte secondary
batteries A to D and K of the Examples include the structure
retainer in both the mixture layer of the positive electrode and
the porous insulating layer. Therefore, even when the active
material of the negative electrode is stimulated to expand, the
structure retainer prevents expansion of the active material of the
negative electrode (especially, expansion of the active material of
the negative electrode in the thickness direction of the active
material layer of the negative electrode). Thus, the nonaqueous
electrolyte solution is prevented form being expelled out of the
mixture layer of the positive electrode, the porous insulating
layer, and the active material layer of the negative electrode.
Accordingly, the number of cycles can be increased, and the cycle
characteristics can be improved.
[0125] Comparing the nonaqueous electrolyte secondary battery A of
Example 1 and the nonaqueous electrolyte secondary battery D of
Example 4, the battery A in which the ceramic granules in the shape
of twigs were used as the structure retainer achieved a greater
number of cycles than the battery D in which the ceramic granules
were spherical. A conceivable reason for this is that the ceramic
granules in the shape of twigs are more likely to be entangled with
one another than the spherical ceramic granules and therefore
improves the strength of the structure retainer.
[0126] Comparing the nonaqueous electrolyte secondary battery I of
Example 5 and the nonaqueous electrolyte secondary battery J of
Comparative Example 5, the nonaqueous electrolyte secondary battery
I achieved a greater number of cycles than the nonaqueous
electrolyte secondary battery J. The capacity is larger but the
expansion rate is higher when an alloy or metal oxide is used as
the active material of the negative electrode than when carbon is
used as the active material of the negative electrode. Thus, when
the separator made of a polyethylene resin is used instead of the
porous insulating layer as in the nonaqueous electrolyte secondary
battery J, the number of cycles greatly decreases. On the other
hand, the nonaqueous electrolyte secondary battery I includes the
structure retainer in both the mixture layer of the positive
electrode and the porous insulating layer. Therefore, even when the
active material of the negative electrode expands, the structure
retainer prevents expansion of the positive electrode and the
porous insulating layer. Thus, the nonaqueous electrolyte solution
is prevented from being expelled out of the mixture layer of the
positive electrode, the porous insulating layer, or the active
material layer of the negative electrode, so that the number of
cycles increases.
[0127] Thus, by employing the structures of Examples 1 to 6,
nonaqueous electrolyte secondary batteries having excellent cycle
characteristics can be provided.
* * * * *